Motionless mixers have a wide variety of industrial uses. They are used to advantage in any context where two or more substances, one of which is usually a fluid, require mixing. Typical applications are for mixing chemicals in industrial processes, mixing multi-part curing systems for adhesives, foams and molding compounds, mixing fuels and gases for combustion, mixing air into water for sewage treatment, etc. In many instances, the thoroughness and efficiency of the mixing has important economic significance. The growing number of uses for motionless mixers has laid emphasis on improvements in their construction and reduction in their cost of manufacture.
Among the objectives of the design of motionless mixers is to provide a desired degree of mixing within the least axial dimension of a conduit and with the least pressure drop. The cost of a motionless mixer installation is directly proportional to its axial dimension. This is because the mixer modules are one of the main items of expense, and the fewer modules that can be used to accomplish a given degree of mixing, the less expensive will be the installation. In addition, the cost of pumping the fluid is significant, and, therefore, it is important to accomplish a given degree of mixing with the least possible pressure drop in the conduit.
A further cost factor relates to the fabrication of the modules themselves. In the modern industrial context, a mixing installation can require many hundreds, or even thousands, of modules. The cost of fabricating such modules can be very significant and, therefore, savings in this area are also sought.
Another factor which can become important is the ease of assembly both at the point of fabrication and in the field.
A final factor is maintenance in the field. The motionless mixer modules should be easy to clean and free from the accumulation of unmixed components of the materials intended for mixing.
In the mixing of substances with motionless mixers, the mixing action is influenced by the viscosity and flow rate of the substances. If the viscosity is low, and the flow rate high, the moment of inertia of the fluid as it changes direction in the conduit, can contribute significantly to the mixing action. Conversely, with highly viscous materials and slow flow rates, the kinetic energy of the materials plays little or no part in the mixing action.
The following criteria have been found important. First, the entire cross-section of the stream, at each stage along the conduit, should receive essentially the same mixing action. Otherwise, some parts will be mixed before others. In many instances mixing causes the viscosity to change, and non-uniformity of flow results. Thus, in any mixing arrangement in which the mixing action does not act substantially simultaneously on all parts of the cross-section of the stream, the mixing action must be continued longer in order to attain a given degree of mixing.
Since motionless mixers, more or less, by definition must be interposed in a moving stream, they usually are arranged to divide the stream into two or more flow paths. It follows that, if uniform mixing across the cross-section of the stream is to be provided, the separate flow paths must be identical both in mixing action and resistance to flow. In addition, motionless mixers should be designed to allow the material to flow as a mass without impeding one part of the stream more than another part. Likewise dead spaces should be avoided. Dead spaces which can allow a portion of the stream to stagnate obviously interfere with mixing. Likewise, the mixer must not have a free path extending along its axis through which fluids can flow in preference to being subjected to mixing.
In general, as the mixing action of a motionless mixer module is increased so as to shorten the axial distance along the conduit to be devoted to the mixing, the changes to the module to accomplish this objective also increase the pressure drop. At some point, the gains due to shortening the mixing column are offset by the requirement for increased pumping pressure, and, therefore, an objective in the design of a motionless mixer is to meet the optimum balance between these two factors.
A further aspect of mixing which needs to be taken into consideration is that with motionless mixers, experience has shown that the most efficient results are obtained if the stream is subjected to a strong mixing action substantially along a line uniformly across its cross-section in a strong mixing zone, and, thereafter, the stream is then allowed to follow an even flow for a finite period in preparation for entrance into a further strong mixing zone.
A well-known motionless mixer which met many of the above criteria is described in Armeniades U.S. Patent No. The Armeinades mixer comprises a series of modules formed out of short twisted helix elements connected orthogonally at their ends to both split the stream, and reverse its helical flow path between each element. In a typical form of the Armeniades structure, the helix of each module is formed to turn the stream 180.degree. and the axial dimension of the modules is 11/2 times the inside diameter of the conduit. Such a module is said to have a "pitch ratio" of 1:1.5, meaning that in an axial dimension of 11/2 times the I.D. of the tube, the helix will twist 180.degree.. With such modules, it was found that a favorable strong mixing zone was established along a transverse line immediately downstream of the entrance point of each module. Also, virtually the entire cross-section of the stream was subjected to this strong mixing action, and, subsequently, the entire stream entered a relaxation zone downstream in preparation for entering a further strong mixing zone. In addition, the Armeniades structure provided precisely equal flow paths on each side of the modules, no local obstructions, and virtually no dead places other than a dead space which might occur along a small line at the ends of the modules if they were not tapered. In addition, although a helix, as used in Armeniades, seems to provide a straight line along its axis along which fluids might flow, in actual fact, with a helix the flow path continuously crosses the axis diagonally, and at no point can the fluid follow a straight line along the axis.
The Armeniades structure, however, has a number of serious drawbacks as follows: first, a helix is difficult to form accurately. A crude helix can be formed by twisting a narrow strip of material, but since the edges of a helix must be substantially longer than the centerline, unless the edges of the material can stretch as the strip is being twisted, it will not form a helix. Even if the edges stretch, strain will be introduced and a true helix is apt not to be formed. These problems are aggravated geometrically as the size of the modules is increased. It is possible to mold modules of the Armeniades type of any size, but molds for this purpose are expensive and so are the parts made from them. Likewise, although theoretically dies can be made in helical form or other compound cured surfaces, so as to permit die stamping modules of the Armeniades type out of flat stock, such an operation would be very expensive.
Accordingly, efforts have been made in the past to cut motionless mixers out of flat stock and then bend them into various shapes without introducing expensive compound curves. Modules made in this way have been used in much the same way as are the Armeniades modules, that is, they are arranged in abutting, end to end relation in a tube. Usually, they are designed to split the stream into two paths and usually they are provided with vanes which cause the fluids to assume a tortuous path designed for mixing. A large number of patents have issued on various forms of such motionless mixers as cited in my prior co-pending application. The prior art static mixers which are most pertinent to the present invention are shown in the patents of Komax Systems, Inc., U.S. patents to King, U.S. Pat. Nos. 3,923,288 and 4,034,965, and the patent of Phillips Petroleum Company, U.S. patent to Crouch, U.S. Pat. No. 3,643,927.
The King mixers have the advantage of not requiring attachment between modules, but otherwise they have a number of drawbacks. First, in the King mixers, the angle between the mixing vanes and the wall of the tube is sharply acute in many areas where the general fluid flow path is across the line between the vane and the tube. This provides a dead space where the material will stagnate and, hence, be poorly mixed. Also in King, except for where the fluid passes his relatively short axially aligned baffles 18, the fluid is subjected to continuous turbulence without any clearly defined strong mixing zone extending on a line across the full cross section of the stream. Also in King there is virtually no relaxation zone in which the stream is forced to follow a helical flow path so as to condition it for entrance into a new strong mixing zone along a transverse line downstream. In addition, King's modules impose a relatively high pressure drop for the amount or mixing they accomplish. Finally, in King, the stream is not split equally and reversed along a transverse line between modules.
Crouch's modules, although composed solely of flat vanes, cause the fluid to take a generally helical path which is equally split at the end of each module. Crouch, however, has no distinct strong mixing action along a transverse line followed by a relaxation zone. In Crouch, as in King, there are acute-angle pockets away from the main flow path of the fluid, in which unmixed materials may stagnate. Further, Crouch's mixing is caused only by the flow of fluid over and along the line of reflex angles centrally of his modules. This provides some mixing but is less efficient than mixing in which the fluid flows transversely to the line of reflex angles, and far less efficient than mixing, in which the fluid flows over the free edge of a vane and meets a cross-flow of a second stream at an angle.
Accordingly, among the objects of this invention is the provision of a motionless mixer which can be manufactured by the mere cutting and bending of flat stock without any compound curves, having vanes for directing the fluid stream in a generally helical path alternately through a strong mixing zone in which virtually the entire cross section of the flowing material receives strong mixing, along a transverse line, followed by a relaxation zone in which the material is directed in a generally helical path in preparation for entrance into a further strong mixing zone where the fluid undergoes a reversal of helix direction along a line further downstream. An additional object is to provide a motionless mixer with such a strong mixing zone in which the materials being mixed are subjected to a sharp change of direction at the free end of a mixing vane. Another object is to provide a flow path for the fluid which generally follows the groove of any included angle so as to continuously flush out any stagnant pockets that might otherwise exist in the grooves formed by such included angles. Still another object is to provide for making such a static mixer by joining two or more sub-components with a minimum number of joints or weldments. A further object is to provide an arrangement of such static mixer modules in which a portion only of a module is provided respectively for fluid entrance and fluid exit, whereby the mixing effect of two modules is achieved with approximately the pressure drop of only one module. Further objects are to provide such motionless mixer modules conveniently in any size and for any size or shape of conduit, and with any desired pitch ratio.